Olfactory glomeruli, which are compartments of neuropil where synaptic interactions between olfactory sensory axons and their target neurons take place, have been known for some years to be induced by the ingrowth of olfactory sensory axons. Studies in the current grant period have revealed that glial cells, the major non-neuronal cells of the nervous system, play an essential role in the olfactory-axon induced formation of olfactory glomeruli in a model system, the antennal lobe of an insect. The proposed research aims to elucidate the role that neuron-glia, and also neuron- neuron, interactions play in the construction of glomeruli, and to further our understanding of the mechanisms underlying these important developmental events. In particular, the proposed experiments will continue to test the hypothesis that glial cells act as intermediaries between the sensory axons and their eventual synaptic targets, i.e. that glial cells are "transformed" by exposure to olfactory axons and that "transformed" glia form the scaffold within which target neurons differentiate glomerular arbors. Toward this end, the experiments will: (a) determine the precise time in the development of glomeruli when glial cells must be present, (b) determine how stable is the "transformation" of glia by olfactory axons, (c) explore the expression of cell-surface and extracellular-matrix markers during development of the glomeruli, (d) determine whether the glia-defined boundaries of glomeruli represent diffusion barriers for ions or for other potential factors influencing differentiation, and (e) discover the extent of glial cell death and determine its significance in the creation of the glomerular scaffolds. In mammals, recent findings suggest that glial cells may play a role in the early compartmentation of the central nervous system in a manner similar to that observed in the antennal lobe of the insect. In the long run, understanding the roles and mechanisms of intercellular interactions in the more accessible insect system will offer insights into interactions in mammalian systems and may reduce the need for experimentation in mammals.